Intensity discrimination is the process of distinguishing one stimulus intensity from another

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Intensity Discrimination
Intensity discrimination is the process of
distinguishing one stimulus intensity from another
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Two types Difference thresholds the two
stimuli are physically separate Increment
thresholds the two stimuli are immediately
adjacent or superimposed
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Fig. 1.1
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Fig. 1.2
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Theory and Practice
Theory
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Fig. 2.7 From Dr. Krafts course Hecht, Shlaer
Pirenne Photon emission follows a Poisson
distribution
To distinguish a flash with a mean of 8 from a
flash with a mean of 9 quanta is impossible! The
distributions overlap almost completely
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Mean of 8, vs. mean of 9
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Mean of 8, vs. mean of 12
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Mean of 8, vs. mean of 16
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Mean of 8, vs. mean of 20
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In a Poisson distribution, the variance is equal
to the mean. The standard deviation (SD) is the
square root of the mean. In a two-alternative
forced-choice task, to reach threshold (75
correct), LT must differ from L by 0.95 SD.
(e.g., ?L 0.95 SD)
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Moreover, as L increases, the minimum ?L also
increases with the
because the variance in a Poisson distribution
equals the mean, so the SD changes with the
square root of the mean
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An ideal observer would follow the
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Theory and Practice
In practice
at low background intensities, human observers
behave as an ideal detector (follow the
deVries-Rose Law)
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Fig. 3.1
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Fig. 3.1
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You always can determine the Weber fraction, even
when Webers Law does not hold
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Fig. 3.2
Webers Law does NOT hold (?L/ L rises as L
decreases)
Webers Law holds
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Both the deVries-Rose and Webers laws fail to
account
for thresholds at high light intensities
Fig. 3.3
The increment threshold data of a rod
monochromat (circles) plotted along
with the theoretical lower limit (deVries-Rose,
dotted line) and the predictions of Webers
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Law (solid line). Luminance values are in cd/m
. (Redrawn from Hess et al. (1990)
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More practical issues How changes in other
stimulus dimensions affect the Weber fraction
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1 Stimulus size the Weber fraction is lower
(smaller) for larger test stimuli
Fig. 3.4
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More practical issues Is a target visible under
certain conditions?
D
Log Weber Fraction,
L/L
2
This is the targets Weber fraction. It is NOT a
threshold
Test Field Diameter
4'
1
121'
0
Is a spot with a particular luminance, relative
to background, visible? It depends on its size.
-1
-2
If the target is 121, it is visible If 4, it is
not visible
-3
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
2
Log Background Intensity, L (cd/m
)
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Need to distinguish between the Weber fraction of
a target vs. the threshold of a viewer. For a
subject or patient viewing a target, if the
subjects Weber fraction is below a line, then
the subjects threshold is better (smaller). If
the Weber fraction of a target is below the line,
the target is NOT visible to someone whose
threshold is on the line.
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The smaller the threshold ?L, the smaller is the
value of the Weber fraction for a given
background L, (only the numerator changes) and
the more sensitive the visual system is to
differences in light intensity.
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Fig. 1.2
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The dinner plate example Plate with luminance
of 0.0102 footlamberts. Background is 0.01
footlamberts ?L is thus 0.0102 0.01
0.0002. ?L/L 0.0002/0.01 0.02 (plate is 2
more intense) From Figure 3-4, can learn that
this is not visible.
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.
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More practical issues Is a target visible under
certain conditions?
D
Log Weber Fraction,
L/L
2
Test Field Diameter
4'
1
Is a spot with a particular luminance, relative
to background, visible? It depends on its size.
121'
0
Threshold Weber fraction for 121 objects
-1
-2
Plates Weber fraction
-3
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
2
Log Background Intensity, L (cd/m
)
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Continuing How changes in other stimulus
dimensions affect the Weber fraction
2 Short-duration flashes are harder to see (are
less discriminable) than long-duration
flashes That is, the threshold ?L increases as
flash duration becomes shorter.
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Sensitivity 1/threshold
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Continuing How changes in other stimulus
dimensions affect the Weber fraction
3 Threshold ?L varies with eccentricity from
the fovea At low luminance levels, threshold is
lowest (sensitivity is highest) about 15-20
degrees from fovea and the fovea is blind At
high luminance levels, threshold is lowest at the
fovea
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This is the basis for visual field tests
Fig. 3.5
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Sensory Magnitude Scales Revisited Using the
just noticeable difference (jnd) to create a
scale for sensory magnitude vs. stimulus
magnitude L threshold ?L LT LT is one jnd
more intense than L. LT threshold ?L LT2 LT2
is one jnd more intense than LT And so on
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Sensory Magnitude
12
10
8
6
4
L
2
0
0
50
100
150
200
2
Stimulus Luminance, L (cd/m
)
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Sensory Magnitude
12
10
L threshold ?L LT LT is one just noticeable
difference (jnd) more intense than L.
8
6
LT
4
L
2
0
0
50
100
150
200
2
Stimulus Luminance, L (cd/m
)
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Sensory Magnitude
12
LT threshold ?L LT2 LT2 is one jnd more
intense than LT and 2 jnds larger than L
10
8
LT2
6
LT
4
L
2
0
0
50
100
150
200
2
Stimulus Luminance, L (cd/m
)
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Sensory Magnitude
12
LTn1
10
LTn
8
When Webers Law holds, the threshold ?Ls keep
getting larger, so 1 jnd is a larger increase in
stimulus luminance
LT2
6
LT
4
L
2
0
0
50
100
150
200
2
Stimulus Luminance, L (cd/m
)
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Fechners Law
Sensory Magnitude
12
10
8
6
4
2
Fechner's Law Log(L)
0
0
50
100
150
200
2
Stimulus Luminance, L (cd/m
)
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Comparing Fechners Law with Stevens Power Law
Fig. 3.6
Stevens Power Law resembles Fechners Law when
the exponent is lt1